An ever increasing demand for higher cellular network data rates raises the challenging question for operators of how to evolve their existing cellular networks to meet this demand. In this regard, a number of approaches are possible: i) increase density of typical macro (large, high power) base stations, ii) increase cooperation between macro base stations, or iii) deploy smaller lower power base stations as an overlay to a macro cellular network in areas where high data rates are needed within the macro network topology.
The 3rd Generation Partnership Project (3GPP) provides technical specifications for a 3rd generation mobile system based on evolved Global System for Mobile Communications (GSM) core networks. In 3GPP, the last option (iii) is commonly referred to as a “Heterogeneous Network” or “Heterogeneous Deployment.” Lower power base station nodes are referred to by a variety of names including “micro”, “pico”, “femto” or “home” base stations (HBS) depending on transmit power and targeted coverage of the low power nodes. Specifically, deployment of femto base stations, or home eNBs (HeNB using LTE terminology), can lead to greater local coverage and throughput, e.g., for in-building users.
HeNB's are often deployed using a closed subscriber group (CSG) in order to restrict authorized users of the HeNB to an identified and approved set of users or user equipment (UE). When closed subscriber group home base stations (CSG HeNB's) are deployed as an overlay to an existing macro cellular network deployment, macro network UEs (MUEs) that are not part of the CSG can be adversely affected by the CSG HeNB. For example, macro user equipment which is not part of the closed subscriber group (non-CSG MUE) but is near or in the coverage area of the CSG HeNB may receive interference from the CSG HeNB. A non-CSG MUE near an edge of its macro cell coverage and receiving a relatively weak downlink (DL) signal from its macro cell base station (e.g., eNB), while simultaneously being close to an CSG HeNB and receiving a relatively strong interfering DL signal from the CSG HeNB, results in reduced DL throughput for the non-CSG MUE. Reduced DL throughput may lead to loss of coverage depending on the strength of the CSG HeNB interferer.
A number of potential approaches may mitigate the interference to non-CSG MUEs within the coverage area of CSG HeNB's. In a CSG HeNB deployment, one approach is to handover the non-CSG MUE from the macro eNB to the HeNB and thus let the non-CSG MUE be served by the CSG HeNB. In a typical CSG HeNB deployment, however, a handover of non-CSG MUEs is generally not possible as the non-CSG MUE is not part of the CSG. Another possible approach employs a form of autonomous power control of the CSG HeNB transmit power based on the received signal power from the non-CSG MUE or on the strongest co-channel interferer as seen by the HeNB. Lastly, another possible approach is to have the HeNB avoid scheduling its UEs on the same radio bearers (RBs) as the non-CSG MUE that is in close proximity to the CSG HeNB. This approach can effectively mitigate the interference but reduces the capacity of the HeNB.
These approaches may potentially reduce the interference and outage as seen by the non-CSG MUEs. However, the CSG HeNB has no direct knowledge of radio bearers (RBs) that non-CSG MUEs are scheduled on or an actual level of interference that the non-CSG MUE is receiving. The received signal strength received from the non-CSG MUE in the uplink (UL) at the CSG HeNB is different from the strength of a DL signal received from the CSG HeNB at the non-CSG MUE, e.g., because of different transmit power levels of base stations and UEs which may also be dynamically controlled. It is therefore difficult for the CSG HeNB to accurately adjust its power to sufficiently reduce the interference to the non-CSG MUE without reducing received signal power as seen by UEs served by the CSG HeNB. In addition, this reduction in received signal power potentially reduces communication quality of the CSG HeNB UEs (HUEs).
What is needed is an approach that can accurately predict channel conditions and expected interference signal power encountered by non-CSG user equipment affected by a CSG node.